Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Astronomy

First Advisor

Nagai, Daisuke

Abstract

Galaxy clusters are the largest coherent structures in the Universe today. With masses of over $10^{14}M_\odot$ and hundreds to thousands of member galaxies, they represent the highest-density peaks of the Universe, sitting at the intersection of large-scale filaments as nodes of the Cosmic Web. In a $\Lambda$CDM Universe, smaller structures collapse first, and form ever larger structures by merging with one another. The story of a galaxy cluster, then, is the story of the Universe - a story of the balance between gravitational collapse, Hubble expansion and, at later times, acceleration by dark energy. Most of a cluster's mass is dark, and most of the baryonic mass is in the diffuse intracluster medium (ICM); only about 3$\%$ of the cluster mass lies in galaxies. Star formation and stellar feedback, whose energy can be comparable to the binding energy of an individual galaxy, are negligible compared to the gravitational potential of the cluster. Cluster cosmology until fairly recently has relied on the relative unimportance of baryonic processes, and treated the baryons simply as observable tracers of the total gravitational potential, dominated by the dark matter. Cluster cosmology and cluster astrophysics, therefore, have remained slightly distinct areas of research. Over the course of my PhD, I have investigated several questions that complicate this simple model. I introduce the standard model of cosmology today, $\Lambda$CDM, and how we understand the formation of cosmic structure. I describe how optical, sub-millimeter, radio, gravitational lensing and X-ray observations probe different components of galaxy clusters, and how these relate to each other. Then, I explain the basic principles of numerical simulations, and the role of cosmological as well as idealised simulations in understanding galaxy clusters. My doctoral work then addresses three poorly understood processes - the role of Active Galactic Nuclei, mergers, and magnetic fields in the evolution of galaxy clusters, and their implications for cluster cosmology.

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